I assume you're referring to the general log/index retention period, not to a single log-file switch, which doesn't actually affect anything on R80.10 & above, as it only impacts the old 'Non-Index' I/S.
Both are on the SmartConsole > Open the Management Server Object > Logs > Storage.
It depends on 2 factors:
1. Daily Log/Indexed-logs retention via: 'Delete index files older than...'
(on R80.40 you can keep/delete both logs & indexed-logs in a more comfortable manner - recommended).
2. Disk-Space ('emergency') threshold for Logs/Indexed-logs retention, which depends on your actual disk-space in Logs partition (/var/log/), via: 'Delete files once threshold is below 'X' available space (default: 5GB==5000Mb).
Example: You have 10GB of Logs & 10GB of Indexed Logs per day. (~700 logs/sec)
Your total logs partition is 1000GB, so you can save around 1000/20 = 50 days of max logs/Indexed-logs, unless configured as less via daily retention.
Hope that's clear.